📑 Topics on this page (Jump to Topic):
📌 1. Work & Energy
- Work W = F·s·cosθ (joule). Work is done only if there is displacement along the force.
- At θ = 90° work is zero — e.g. walking horizontally with a load on the head, centripetal force in circular motion, pushing a wall (no displacement).
- Kinetic energy KE = ½mv²; doubling velocity → KE becomes four times.
- Potential energy PE = mgh; stretched spring, water held in a dam, a drawn bow.
- Conservation of energy: energy can neither be created nor destroyed, only changed in form. For a falling body PE + KE stays constant.
| Device | Energy conversion |
|---|---|
| Dynamo / generator | Mechanical → electrical |
| Electric motor | Electrical → mechanical |
| Cell / battery | Chemical → electrical |
| Solar cell | Light → electrical |
| Microphone | Sound → electrical |
| Loudspeaker | Electrical → sound |
| Electric bulb | Electrical → light + heat |
| Photosynthesis | Light → chemical |
📌 2. Power
- Power P = W/t, unit watt (J/s). 1 kW = 1000 W; 1 hp ≈ 746 W.
- P = F × v — so on a slope a vehicle is run in low gear (more force, less speed).
- Efficiency = (output work / input work) × 100%; no machine is 100% efficient.
📌 3. Simple Machines & Levers
A lever has three points: fulcrum (F), load (L) and effort (E). Whichever is in the middle decides the class.
| Class | In the middle | Examples |
|---|---|---|
| First | Fulcrum | Scissors, see-saw, pliers, beam balance, hand pump |
| Second | Load | Nutcracker, wheelbarrow, bottle opener, boat oar |
| Third | Effort | Tongs, human forearm, fishing rod, broom |
- Mechanical advantage = load / effort. Second class always has MA > 1; third class always MA < 1.
- Other simple machines: pulley, inclined plane (ramp), screw, wedge (axe), wheel and axle.
⚠️ Exam Traps:
- Pushing a wall for hours still means zero work in physics (no displacement).
- A dynamo produces electricity; a motor uses it — do not swap them.
- Tongs = third class, nutcracker = second class, scissors = first class.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. The SI unit of work is: Practice
Correct answer: (C)
1 J = 1 N × 1 m.
1 J = 1 N × 1 m.
Q2. If the velocity of a body is doubled, its kinetic energy becomes: Practice
Correct answer: (D)
KE ∝ v²; 2² = 4.
KE ∝ v²; 2² = 4.
Q3. A dynamo converts: Practice
Correct answer: (B)
A dynamo produces electricity by electromagnetic induction.
A dynamo produces electricity by electromagnetic induction.
Q4. A porter walking on a level road with a load on his head does how much work against gravity? Practice
Correct answer: (D)
Force is upward, displacement horizontal — θ = 90°, cos 90° = 0.
Force is upward, displacement horizontal — θ = 90°, cos 90° = 0.
Q5. Scissors are a lever of which class? Practice
Correct answer: (A)
The fulcrum (rivet) is in the middle.
The fulcrum (rivet) is in the middle.
Q6. A nutcracker is a lever of which class? Practice
Correct answer: (C)
The load (nut) lies between the fulcrum and the effort.
The load (nut) lies between the fulcrum and the effort.
Q7. The human forearm acts as which class of lever? Practice
Correct answer: (A)
The effort (biceps) is between the fulcrum (elbow) and the load (palm).
The effort (biceps) is between the fulcrum (elbow) and the load (palm).
Q8. The SI unit of power is: Practice
Correct answer: (C)
1 W = 1 J/s.
1 W = 1 J/s.
Q9. Water stored in a dam has: Practice
Correct answer: (B)
Due to height, PE = mgh; when it falls this becomes kinetic and then electrical energy.
Due to height, PE = mgh; when it falls this becomes kinetic and then electrical energy.
Q10. PE of a 2 kg body at 5 m height (g = 10 m/s²) is: Practice
Correct answer: (D)
mgh = 2 × 10 × 5 = 100 J.
mgh = 2 × 10 × 5 = 100 J.
Q11. A solar cell converts: Practice
Correct answer: (B)
A solar cell (usually silicon) works by the photovoltaic effect.
A solar cell (usually silicon) works by the photovoltaic effect.
Q12. A machine does 500 J of work in 10 s. Its power is: Practice
Correct answer: (D)
P = W/t = 500/10 = 50 W.
P = W/t = 500/10 = 50 W.
Q13. According to the law of conservation of energy: Practice
Correct answer: (A)
The total energy of an isolated system stays constant.
The total energy of an isolated system stays constant.
Q14. A loudspeaker converts: Practice
Correct answer: (C)
A microphone does the reverse.
A microphone does the reverse.
Q15. Why does a truck driver use a low gear on a slope? Practice
Correct answer: (B)
P = F × v; at fixed power lower v gives higher F.
P = F × v; at fixed power lower v gives higher F.
Q16. In which lever is the mechanical advantage always greater than 1? Practice
Correct answer: (B)
In second-class levers the effort arm is always longer than the load arm.
In second-class levers the effort arm is always longer than the load arm.
Q17. A drawn (stretched) bow stores: Practice
Correct answer: (A)
Energy stored by change of shape is potential; it becomes kinetic when the arrow is released.
Energy stored by change of shape is potential; it becomes kinetic when the arrow is released.
Q18. How many joules are there in 1 kWh? Practice
Correct answer: (A)
1000 W × 3600 s = 3.6 × 10⁶ J.
1000 W × 3600 s = 3.6 × 10⁶ J.
Q19. Tongs are a lever of which class? Practice
Correct answer: (D)
The effort is applied in the middle.
The effort is applied in the middle.
Q20. For a ball falling freely (air resistance negligible), which is true? Practice
Correct answer: (C)
Mechanical energy is conserved — potential turns into kinetic.
Mechanical energy is conserved — potential turns into kinetic.